1. Field of the Invention:
This invention relates generally to fission product contamination of a reactor coolant, and more particularly to a method for determining the concentration of fission products in the reactor coolant of a liquid metal cooled nuclear reactor.
2. Description of the Prior Art:
In general, a nuclear reactor produces heat by fissioning of nuclear materials which are fabricated into fuel elements and assembled within a nuclear core. In commercial nuclear reactors, the heat produced thereby is used to generate electricity. A liquid metal cooled nuclear reactor is an example of such a commercial reactor.
In this type of reactor, a reactor coolant such as liquid sodium, circulates through a primary heat exchange system whereby the nuclear heat is removed from the core and transferred to a secondary heat exchange system. It is within the secondary heat exchange system that the heat is transferred into steam which is ultimately used to generate electricity by conventional turbine-generator apparatus. The primary heat exchange system which mostly concerns this application, includes the aforementioned nuclear core, a reactor vessel for housing the core, a heat exchanger, a circulating pump, and piping interconnecting these apparatus.
In the type of nuclear reactor considered herein, the fuel elements usually comprise elongated sealed tubes or "cladding" filled with nuclear fission material, such as plutonium 239 or enriched uranium, in the form of cylindrical pellets. Hence, fission products and fission gases which are produced as a result of the nuclear reaction, during reactor operation remained contained within the sealed cladding. There is the possibility, however, that a breach of the cladding may occur and allow these fission products to escape into the reactor coolant flowing past the fuel elements. As long as the concentration of these fission products in the reactor coolant remains low, no problem arises. But, should a relatively large number of fuel elements fail, which is highly unlikely, the fission product contamination of the reactor coolant could achieve harmful levels. High concentrations of fission products cause the reactor coolant itself to become radioactive. And, because the fission products readily precipitate out of solution they cause the heat exchanger, the circulating pump, the interconnecting piping and other equipment to become radioactive. This, of course, has the effect of substantially interfering with personnel access to these components for such necessary tasks as equipment repair and maintenance.
In the prior art, a relatively simple and direct method of determining fission product contamination of the reactor coolant has been utilized. Namely, the level of radioactivity of the reactor coolant was measured by a gamma spectroscopy. The determination of the level of fission product contamination was necessarily limited, therefore, to relatively low sensitivities. Presently, the sensitivity of the prior art methods limits the detection of nuclide activities to a level of 10.sup..sup.-5 microcuries per mililiter. As this level of radioactivity is relatively high, it only indicates that a substantial failure within the nuclear core has occurred. It does not indicate a relatively minor failure. Although it would be extremely advantageous to learn of the occurrence of a relatively minor or a less significant failure within the nuclear core, such a determination is not possible with the prior art method. Thus, there exists a continuing need to improve upon methods for determination of fission product contamination of a coolant of a nuclear reactor.